The javadoc [1] for javax.ws.rs.core Configurable interface says that:

"An existing property can be removed by assigning a null value to the property. "

By client.property(some_property, null) this is not possible.
ResteasyProviderFactory.property() method only adds the properties, but has no way to remove it if the value of the property is null.

[1] http://docs.jboss.org/resteasy/docs/3.0.9.Final/javadocs/index.html